Thermosetting adhesives have been used for many years where durable weather resistant bonds are required. Most of these are based on phenol-formaldehyde condensation products or phenol-formaldehyde polymers further modified by addition of a dihydric aromatic compound such as resorcinol. Occasionally resorcinol-formaldehyde polymers have been used as adhesives but these are less common because of their high cost. (The phenol-resorcinol-formaldehyde, or PRF, adhesives have the advantage that they cure to infusible and insoluble thermoset polymers at room temperatures.) Other adhesives that will cure very rapidly at room temperature, such as those based on condensation products of formaldehyde and phenol with aromatic amine compounds; e.g. m-aminophenol, diaminonaphthalenes or hydroxyaminonaphthalenes, and conjugated nitrogen containing heterocycles such as pyrroles or diaminopyridines have not found commercial acceptance for a number of reasons. These include high cost and toxicity of some ingredients. Adhesives which are exemplary of these very rapid curing types are described in U.S. Pat. Nos. 3,784,515, 3,786,025, and 3,784,514 respectively.
The method of maling phenol-resorcinol-formaldehyde adhesive compositions is well known in the art. Typically, a phenol-formaldehyde resole is first formed. This may be condensed in the presence of an alkaline catalyst such as sodium hydroxide or sodium carbonate or with an ortho-directing catalysts such as calcium or zinc acetate. At some point after the initial condensation, resorcinol is added in an amount so that the ultimate ratio of formaldehyde to aromatic hydroxy compounds is less than unity, typically about 0.65 to 0.75. U.S. Pat. Nos. 2,385,372 and 2,414,414 are typical of processes making the alkaline condensed resins while U.S. Pat. Nos. 3,492,263 and 4,061,620 are directed to ortho-condensed phenol-resorcinol adhesive resins. These resins are mixed with a hardener at the time of use. Hardeners contain allylene group donors and most typically include fornalin, paraformaldehyde, or .alpha.-polyoxymethylene mixed in a powdered solid system or in liquid suspension with an inert filler
Phenol-resorcinol-formaldehyde adhesives are readily available from several commercial sources. The commercial products are sold in a number of grades for different applications; e.g., finger jointing and laminating. All of these are basically similar although they may have minor differences in such properties as water dilutability, viscosity, resorcinol to phenol ratio, pot life after hardener addition, compatibility with solid or liquid hardener systems, etc. While there are PRF resins that are preferred for use with the present invention, many if not most of those presently on the market would be acceptable. PRF adhesive resins per se form no part of the present invention.
Environmental concerns affecting both public and private forest lands, and poor stewardship on many forest lands over the past century, have greatly reduced the available supply of timber for the manufacture of lumber and other forest products. In many cases it is now necessary to use lumber formed from smaller pieces glued together using finger or scarf joints and/or edge gluing or lamination simply because the large logs necessary to cut it in solid sawn form are not available. Unfortunately, gluing technology has not evolved at the same rate as the need for utilization of small and low grade logs.
One particular need of wood products manufacturers has been the development of a rapid curing PRF adhesive. There are many ways in which the ability to rapidly bond wood would be beneficial to forest products producers. Until a short time ago radio frequency heating was the only method for rapidly bonding wood with a PRF adhesive. This system failed to produce reliable bonds when moisture content of the wood was above about 16%. Recently one successful process suitable for rapidly bonding wetter wood has become available. This is described in European Patent Application 493,010. In this process a PRF resin with added hardener is allowed to advance somewhat to a rather high viscosity before it is placed on one of the members to be joined. A cure promoter chemical, selected from groups such as ammonia or amine derivatives, low molecular weight carboxylic acids, or Lewis acids, is placed on the opposite surface to be mated. Aqua ammonia of about 28-30% concentration appears to be a preferred cure promoter. The members are then clamped with sufficient pressure to cause mixing of the cure promoter and resin. Within several minutes the resulting joint has developed sufficient strength to be handled. Further cure until the ultimate joint strength is developed takes place over a matter of hours.
While the particular adhesive system just noted is new, (these so called "honeymoon" systems, in which reactive components are placed separately on opposite sides of a joint, have been known for many years.) The first disclosure to the present inventors knowledge in which a PRF honeymoon resin formulation was used is found in U.S. Pat. No. 2,557,826. U.S. Pat. No. 3,802,986 describes a PRF honeymoon system specifically adapted for finger jointing short pieces of lumber. In some honeymoon systems, such as the one noted in the above European patent application, the resin is placed on one side of the joint and the hardener or an accelerator of some sort on the other. In others, both components are adhesives in their own right and will cure to form bonds even if mixing is poor when the two sides are brought together. For many reasons, some valid and others not, honeymoon adhesives have never found wide acceptance. Despite certain problems such as emission of dangerous ammonia fumes, the ability of the process in the above noted European patent application to glue high moisture content woods may help to overcome this past reluctance to use honeymoon systems.
The present invention represents an improvement in the use of rapid curing resin systems in that it has the many benefits of a honeymoon system in achieving rapid cure times but has the particular advantage of requiring application of the adhesive to only one of the surfaces being bonded.